It's actually a lot simpler than that. All you need is a normal track hub (with 120mm spacing) and two 3mm spacers. Put one spacer on each side of the hub, and you have a 126 spaced hub. The wheel is built centered and with zero dish. You don't need uneven spacers with a real track hub, the reason yours is off center is because it is a converted multispeed wheel.

IRO sells formula hubs (and built wheels) and will space them to 126 for you if you wish. But almost any track hub and 2 M9 washers from the hardware store will work too.